This invention relates to a process for depositing fluorocarbonsulfonic ionomers on a support. This invention particularly relates to a process for preparing fluorocarbonsulfonic ionomer membranes and catalysts.
Fluorocarbonsulfonic ionomers are polymers that are substantially fluorinated and have pendant chains containing sulfonic acid or salt groups. Fluorocarbonsulfonic acid polymers are an important class of fluorocarbonsulfonic ionomers. These ionomers are normally produced by copolymerization of a fluorocarbonsulfonyl fluoride polymer precursor, followed by conversion to the ionomer. Methods for the production of monomers, the polymerization and the conversion are well known in the art of preparing polymers.
Fluorocarbonsulfonic ionomers are known to be useful as ion exchange media as well as for preparing gas separation membranes, fuel cell membranes, electrodes, and chlor-alkali membranes. Fluorocarbonsulfonic acid polymers are very strong acids and are also known to be useful as strong acid catalysts. In catalytic uses, fluorocarbonsulfonic acid polymers are known to catalyze many different reactions: hydration of olefins and epoxides, dehydration of alcohols, alkylation and acylation of paraffins, and the like.
It is known in the art to prepare fluorocarbonsulfonic ionomer articles by several methods. For example, thermoplastic sulfonyl fluoride polymer can be extruded into tubing and converted to the sulfonic ionomer form. But extrusion is not always an advantageous method of preparing such articles. Equipment for extrusion can be both expensive and difficult to handle. In addition, the mechanical properties of fluorocarbonsulfonic ionomer tubing are such that extruding very thin tubes can be impractical.
Another means of preparing fluorocarbonsulfonic ionomer articles is to melt deposit thermoplastic sulfonyl fluoride polymer onto a support and convert the surface to the sulfonic acid form. This process also requires specialized equipment to form the thermoplastic into the desired shape. For catalytic uses, only the surface is used.
A more preferable means of preparing fluorocarbonsulfonic ionomer articles is disclosed by U.S. Pat. No. 4,661,411 to Martin, et al. Therein it is disclosed that fluorocarbonsulfonic ionomer articles can be prepared by depositing a fluorocarbonsulfonic ionomer onto a substrate from a solution by soaking a substrate with a solution that contains a fluorinated polymer having pendant chains containing sulfonic acid groups, removing the solvent from the mixture, and heating the remaining composition to above the glass transition temperature for a time sufficient to render the ionomer insoluble. This process provides advantages including a greater utilization of the polymer. This process is not without problems. It would be desirable in the art to prepare articles with insoluble ionomer layers prepared by means of a process which is substantially faster and utilizes lower temperatures than conventional processes.